Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
Lithographic printing can be further divided into two general types: wet lithographic printing (conventional lithographic printing) and waterless lithographic printing. In wet lithographic printing plates, the ink-receptive areas consist of oleophilic materials and the ink-repelling areas consist of hydrophilic materials; fountain solution (consisting of primarily water) is required to continuously dampen the hydrophilic materials during printing operation to make the non-image areas oleophobic (ink-repelling). In waterless lithographic printing plates, the ink-receptive areas consist of oleophilic materials and the ink-repelling areas consist of oleophobic materials; no dampening with fountain solution is required.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a radiation-sensitive coating deposited on the substrate, the substrate and the radiation-sensitive coating having opposite surface properties (such as hydrophilic vs. oleophilic, and oleophobic vs. oleophilic). The radiation-sensitive coating is usually a radiation-sensitive material, which solubilizes or hardens upon exposure to an actinic radiation, optionally with further post-exposure overall treatment. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate. Conventionally, the actinic radiation is from a lamp (usually an ultraviolet lamp) and the image pattern is generally determined by a photomask (called the film) which is placed between the light source and the plate. With the advance of laser and computer technologies, laser sources have been increasingly used to directly expose a printing plate which is sensitized to a corresponding laser wavelength; photomask is unnecessary in this case. In addition to presensitized plates, press-ready plates can be prepared by direct transferring an external material onto the substrate according to digital imaging information using technologies such as electrophotography (or xerography) and inkjet printing (with or without further curing process), wherein the transferred material and the substrate exhibit substantially opposite surface properties (affinity vs. repellence) for at least one printing liquid selected from the group consisting of ink and an abhesive fluid for ink. For example, for wet plates, the substrate can be hydrophilic and the transferred material can be oleophilic; and for waterless plates, the substrate can be oleophilic and the transferred material can be oleophobic.
One of the more serious problems which can afflict lithographic printing plates is the migration of certain chemical species from the radiation-sensitive layer to the substrate or from the substrate to the radiation-sensitive layer, causing undesirable press performance. For example, in a wet printing plate with hydrophilic substrate, it is well known that migration of chemical species from the radiation-sensitive layer to the substrate can cause loss of hydrophilicity, leading to toning or scumming of the plate (Ink is received in the non-image areas.). In a wet printing plate with silicate coated substrate, migration of certain species (possibly alkaline residues) into the radiation-sensitive layer can effect a certain deterioration of the radiation-sensitive layer during storage (as discussed in U.S. Pat. No. 4,153,461). In addition to migration of chemical species from the radiation-sensitive layer to the substrate, high humidity and other environmental species (such as a solvent or an acid) can also effect the deterioration of the substrate, causing rust or loss of desired surface properties. Chemical reactions between functional groups in the radiation-sensitive layer and functional groups on the substrate at certain conditions (such as higher temperature and humidity) can also lead to undesirable surface properties of the substrate.
For wet printing plates, the above cross-contaminations are especially harmful because of the great propensity for hydrophilic surface to deteriorate. In the manufacture of wet lithographic printing plates, it is well known to coat on the support an insoluble hydrophilic barrier layer which forms the hydrophilic substrate surface of the plate. The barrier layer is utilized primarily to improve the hydrophilicity of the substrate and to minimize contamination and attack of the substrate by chemical species from the radiation-sensitive layer and from the environment. Since such a hydrophilic barrier layer is insoluble in press chemicals, such as fountain solution, ink, developer and press cleaner, it provides consistent hydrophilicity for the background areas of the plates during press operation. Among the various solid materials used for lithographic printing plate supports including metals, plastics and paper, aluminum foil is the most commonly used substrate. For wet lithographic printing plates having an aluminum support, many different materials have been proposed for use in forming such a hydrophilic barrier layer. The hydrophilic barrier layer can be directly applied to the surface of the aluminum sheet material or the aluminum can be grained and/or anodized prior to the application of the hydrophilic coating. Examples of materials useful in forming such hydrophilic coatings are polyvinyl phosphonic acid, polyacrylic acid and polybasic organic acid and their salts, polyacrylamide, copolymers of vinyl phosphonic acid and acrylamide, and silicates. These materials are generally applied to the aluminum surface by dipping the aluminum sheet in a solution of these materials at a certain temperature or by electrochemical deposition, followed by thorough rinse and drying. Hydrophilic coatings which are utilized to form lithographic plate substrate surfaces have been described in various patents, as cited in U.S. Pat. No. 5,368,974 (Walls, et al). Some most representative patents are outlined below.
U.S. Pat. No. 2,714,066 (Jewett, et al) describes formation of an insoluble (i.e., insoluble in fountain solution, ink, developer and press cleaner) hydrophilic layer on aluminum surface through thermal reaction of silicate solution and aluminum surface.
U.S. Pat. No. 3,181,461 (Fromson) describes formation of an insoluble hydrophilic layer on an anodized aluminum surface through thermal reaction of a silicate solution and aluminum oxide coating.
U.S. Pat. No. 3,658,662 (Casson, Jr. et al) describes formation of an insoluble hydrophilic layer on a metal plate through electrochemical anodization in a silicate solution.
U.S. Pat. No. 3,902,976 (Walls) describes formation of an insoluble hydrophilic layer on an aluminum surface by first anodizing the aluminum in an acidic solution to form an aluminum oxide film and then anodizing the oxide film with a silicate solution.
U.S. Pat. No. 4,153,461 (Bergauser, et al) describes formation of an insoluble hydrophilic layer on an anodized aluminum surface through thermal reaction of the aluminum oxide with polyvinyl phosphonic acid.
U.S. Pat. No. 4,399,021 (Gillich, et al) describes formation of an insoluble hydrophilic layer on a metal plate through electrochemical anodization in a water-soluble polybasic organic acid (polyvinyl phosphonic acid being preferred) solution.
U.S. Pat. No. 5,368,974 (Walls, et al) describes formation of an insoluble hydrophilic layer on an aluminum plate through thermal reaction or electrochemical anodization of the aluminum plate with a copolymer of vinyl phosphonic acid and acrylamide.
U.S. Pat. No. 3,860,426 (Cunningham, et al) describes a hydrophilic subbing layer, coated from an aqueous solution of a water-soluble salt of a metal (such as calcium) and a water-soluble hydrophilic cellulosic compound, which is interposed between an anodized aluminum and a radiation-sensitive coating. The anodized aluminum was prepared according to U.S. Pat. No. 3,511,661 (issued May 12, 1970 to Rauner, et al, and disclaimed Oct. 15, 1974). This anodized aluminum surface has micropore openings of about 200 to 750 A and aluminum oxide layer coverage of about 10 to 200 mg/m.sup.2, and are anodized from ungrained or mechanically grained aluminum. The interlayer has a coverage of 2 to 15 mg/ft.sup.2. According to the patent, "the hydrophilic coating is coated over the porous surface in a subbing amount permitting the peaks of the surface to extend above the coating." Apparently, the hydrophilic interlayer fills the micropores of the anodized aluminum surface and also forms a layer on the surface at a thickness thin enough to allow some surface peaks to extend above the coating.
U.S. Pat. No. 4,427,765 (Mohr) describes coating onto an anodized aluminum base (followed by washing and drying) a complex-type product obtained by reacting a water-soluble organic polymer having acid functional groups containing phosphorus or sulfur with a salt of an at least divalent metal cation, to form an insoluble hydrophilic layer. This insoluble hydrophilic layer is further coated with a radiation-sensitive layer.
Formation of a non-polymeric hydroxy-substituted organic acid interlayer on an anodized metal substrate (followed by washing and drying) before coating a radiation-sensitive layer is described in U.S. Pat. No. 4,467,028 (Huang, et al). According to the patent, "the anodized metal substrate is contacted with the acid solution for a time sufficient to form an interlayer, which is probably little more than a monomolecular layer, on the substrate." Clearly, this interlayer formed on the substrate surface is water-insoluble.
In lithographic printing plates based on silver salt diffusion transfer process comprising a base sheet, an imaging receiving layer having a nucleating agent and a silver halide emulsion layer, incorporation of water-soluble salts into the imaging receiving layer is described in U.S. Pat. No. 3,552,315 (Ormsbee, et al); post-treatment of the anodized aluminum foil with an aqueous solution containing one or more organic compounds having at least one cationic group to improve adhesion between the imaging receiving layer and the aluminum base is described in U.S. Pat. No. 5,633,115 (Jaeger, et al).
A tap water developable lithographic printing plate having a radiation-sensitive water-soluble layer interposed between a hydrophilic substrate and an oleophilic radiation-sensitive layer is described in U.S. Pat. No. 4,104,072 (Golda, et al).
An on-press developable lithographic printing plate having a radiation-sensitive hydrophilic water-insoluble layer between a hydrophilic substrate and an oleophilic radiation-sensitive layer is described in U.S. Pat. Nos. 5,258,263 and 5,407,764 (Cheema, et al).
While the above approaches are beneficial in improving certain aspects of the printing plates, none of the approaches can be used in preparing lithographic printing plates without limitation.
The hydrophilic coatings, with or without radiation-sensitive layers, have limited shelf-life (usually one or two years), will deteriorate prematurely if exposed to extreme environmental conditions such as higher temperature and humidity, or will deteriorate if contacted with certain chemical species. In formulating radiation-sensitive layer, certain otherwise beneficial chemical ingredients (such as epoxy resins) often have to be avoided because of their propensity to cause toning or scumming on these hydrophilic substrates.
In the case of silver halide diffusion transfer lithographic printing plates, the water-soluble salts are either incorporated in the imaging receiving layer or are used as adhesion promoter. Cross-contamination issues are not addressed.
For the plates having a radiation-sensitive water-soluble inner layer or a radiation-sensitive water-insoluble hydrophilic inner layer over-coated with a radiation-sensitive top layer, migration of certain chemical species (such as monomers) of the inner layer to the substrate can cause deterioration of the substrate (such as loss of hydrophilicity).
Therefore, there is a continuing need for improving the stability of the hydrophilic coating, minimizing cross-contamination between the hydrophilic substrate and the radiation-sensitive layer, minimizing deterioration of the substrate by environmental species, better tolerance of the substrate in selecting chemicals for formulating radiation-sensitive layer, and better release capability of the radiation-sensitive coating in non-hardened areas while maintaining good adhesion between the radiation-sensitive layer and the substrate in the hardened areas.
Waterless lithographic printing plate constructions disclosed in the patent literature include plates comprising an oleophilic substrate, a radiation-sensitive interlayer and an oleophobic surface coating, and plates comprising an oleophilic substrate and an oleophobic radiation-sensitive coating. Examples of waterless printing plates with an oleophilic substrate having an oleophobic radiation-sensitive coating thereon are U.S. Pat. Nos. 3,997,349, 4,074,009, and 4,508,814. In waterless printing plates with an oleophilic substrate having an oleophobic radiation-sensitive coating thereon, migration of the oleophobic species in the radiation-sensitive layer to the substrate or incomplete removal of radiation-sensitive layer in the non-hardened areas could lead to poor ink receptivity on the developed substrate. Therefore, there is a need for minimizing cross-contamination between the substrate and the radiation-sensitive layer.
On-press developable lithographic printing plates have been disclosed in the literature. Such plates can be developed on press with ink and/or fountain solution. After exposure, the plates can be directly put on press to be developed during the initial prints and then to print out regular printed sheets. On-press developable plates comprising a substrate, a radiation-sensitive water-insoluble hydrophilic layer and an overlaying radiation-sensitive oleophilic layer are disclosed in U.S. Pat. Nos. 5,258,623 and 5,407,764 (Cheema, et al). On-press developable plates comprising a hydrophilic substrate and an oleophilic radiation-sensitive layer are disclosed in U.S. Pat. No. 5,561,029 (Fitzgerald, et al) and U.S. Pat. No. 5,616,449 (Cheng, et al). On-press developable waterless lithographic plates comprising an oleophilic substrate and an oleophobic radiation-sensitive layer are disclosed in U.S. Pat. No. 3,997,349 (Sanders). Because no regular developer and/or gum solution are used, these plates are more prone to background toning and/or ink scumming. Any deterioration on the substrate will have more harmful effect on these plates than on conventional plates. Therefore, for lithographic printing plates to be developed on press, there is a need to reduce contamination of the substrate by chemical species from the radiation-sensitive layer or from the environment and to improve release capability of the radiation-sensitive layer in non-hardened or solubilized areas.